The present invention relates to a method of forming a flattened interlayer insulating film covering a wiring layer of a semiconductor integrated circuit, and a method of manufacturing a semiconductor device.
In recent years, in the field of the semiconductor integrated circuit device, the density of a semiconductor integrated circuit is being prominently increased. In some cases, a multi-layer wiring including several layers is formed within the semiconductor integrated circuit. Since the wiring layer is formed of aluminum in many cases, a strong demand has been directed to the development of a method of forming a flattened interlayer insulating film at a low temperature, e.g., 500.degree. C. or less.
FIG. 1 shows a related method of flattening an insulating film. In this method, a phosphorus-containing insulating film is formed by a thermal CVD (Chemical Vapor Deposition) method or a plasma-enhanced CVD method, followed by heating the phosphorus-containing insulating film so as to fluidize and flatten the insulating film. Also there are an etch back method shown in FIG. 2 and a CMP (Chemical Mechanical Polishing) method shown in FIG. 3. In these related etch back method and the CMP method, the irregularity on the surface of the insulating film is eliminated by etching and polishing, respectively.
In the flattening method based on fluidization by heating, a BPSG film 4 is formed as shown in FIG. 1A by using any of the reactant gases below. In the case of the thermal CVD method, any of the groups of the reactant gases given below are used:
(1) SiH.sub.4 +PH.sub.3 +B.sub.2 H.sub.6 +O.sub.2 (PH.sub.3 : Phosphine PA1 (2) TEOS+TMOP+TMB or TEB+O.sub.2 or O.sub.3 TEOS: tretraethoxysilane (Si(OC.sub.2 H.sub.5).sub.4), TMOP: trimetylphosphate (PO(OCH.sub.3).sub.3)) PA1 (1) SiH.sub.4 +PH.sub.3 +B.sub.2 H.sub.6 +O.sub.2 PA1 (2) TEOS+TMOP+TMB or TEB+O.sub.2 PA1 (1) SiH.sub.4 +O.sub.2 (thermal CVD method or plasma-enhanced CVD method) PA1 (2) TEOS+O.sub.2 or O.sub.3 (thermal CVD method) PA1 (3) TEOS+O.sub.2 (plasma-enhanced CVD method) PA1 (2) With reference to the BPSG film containing P.sub.2 O.sub.5 and SiO.sub.2, it theoretically exhibits an eutectic point of 850.degree. C. at a composition of 20 to 80% of P.sub.2 O.sub.5, and a melting point of P.sub.2 O.sub.5 itself is a key to a fluidizing temperature of the BPSG film. PA1 (3) P.sub.2 O.sub.3 exhibits a melting point prominently lower than that of P.sub.2 O.sub.5 as shown in the following.
Alternatively, in the case of the plasma-enhanced CVD method, any of the groups of the following reactant gases are employed.
Concerning this method, the particular technique is disclosed in, for example, a publication "Williams, D. S. and Dein, E. A.: J. Electrochem. Soc., 134,: 657, 1987, Levin, R. M. and Evans -Lutterodt, K.: J. Vac. Sci. Thechnol., BI, 1:54, 1983, Sato, J. and Maeda, K.: Extended Abstract of Elecrochem. Soc. Spring Meeting: 31, 1971".
Thereafter, the formed BPSG film 4 is heated at a temperature of about 850.degree. C., as shown in FIG. 1B, to fluidize and flatten the film 4. It should be noted that in the case of a PSG film, the film is formed by the thermal CVD method or the plasma-enhanced CVD method using a reactant gas other than the boron-containing gases (B.sub.2 H.sub.6, TMB or TEB) among the above described reactant gases, and then the film is heated at a temperature below 1000.degree. C. to fluidize and flatten the film.
In the case of the method of flattening the film by etching or CMP method, an NSG film (Nondoped Silicate Glass film) 5 is first formed by the thermal CVD method or the plasma-enhanced CVD method using the following reactant gases as shown in FIGS. 2A and 3A, and then the film is flattened.
In the etch-back method, as shown in FIG. 2B, the resist film 6 is formed on the NSG film 5 by a spin coating method, followed by flattening the resist film 6. Then, as shown in FIG. 2C, the film is subjected to an etching that is performed from the upper portion of the film 5, thus forming the flattened NSG film 5a. Moreover, in the CMP method, as shown in FIG. 3B the NSG film 5 is formed and, then polished to obtain the flattened NSG film 5b.
It should be noted that in the foregoing FIGS. 1 to 3, reference numeral 1 denotes a semiconductor substrate. Reference numeral 2 denotes an underlying insulating film. Further, each of reference numerals 3a and 3b denotes a wiring layer formed on the underlying insulating film 2.
In the flattening method using the etch-back method and the CMP method, the film is not heated unlike the flattening method utilizing thermal fluidization. Thus, the etch-back method and the CMP method are effective when that flattening of the film must be performed at a low temperature. Nevertheless, when voids are formed in concave portions between the wiring layers 3a and 3b and in the other concave portions immediately after the formation of the original insulating film 5, these voids remain as they were even after the flattening treatment as shown in FIGS. 2 and 3.
Methods of forming the insulating film providing a good burying property includes, for example, a high density plasma-enhanced CVD method, a ordinary plasma-enhanced CVD method, a normal pressure thermal CVD method, and an SOG (Spin-On-Glass) coating method. However, since flattening using these methods does not utilize the thermal fluidity of the film, when a space between the wiring layers is particularly narrowed by a high integration level of the device, it will be difficult to perfectly bury the concave portion therebetween.
On the other hand, since the film surface flattening method in which the film is flattened by heating and fluidizing the film utilizes the thermal fluidity of the film, it can be expected that the concave portion can be perfectly buried as shown in FIG. 1. At present, particularly, the BPSG film (boro-phospho silicate glass film) 4 is often used for such a purpose. However, in order to fluidize the BPSG film 4, the BPSG film 4 must be heated at least at a temperature higher than 850.degree. C., and this BPSG film cannot be used as the underlying film of the wiring layers 3a and 3b as well as the interlayer insulating film 4, which must be formed at a low temperature.
In this case, if concentrations of phosphorus and boron are set at high level, a temperature for fluidizing the film (hereinafter referred to as a fluidizing temperature) can be decreased to some extent. However, this is unsatisfactory yet, resulting in a new problem in that stability of the interlayer insulating films 2 and 4 and resistance to humidity thereof are degraded. Note that the PSG film requires a fluidizing temperature approximately identical to that for the BPSG film and the above described problem is produced.
Moreover, as an insulating film which requires a low fluidizing temperature, a GBPSG film obtained by adding GeO.sub.2 to the BPSG film has been developed. However, with reference to the GBPSG film, the fluidizing temperature can be lowered to only about 750.degree. C., and it is difficult to apply the BPSG film to the underlying film and the interlayer insulating film which requires a lower fluidizing temperature.